In a cellular system in which a predetermined area is entirely covered by a plurality of base stations, a preselected number (for example, 7 pieces) of frequencies are repeatedly utilized, so that a two-dimensional service is available.
In such a so-called “POINT TO MULTI-POINT type” communication system (will be referred to as a “P-MP type communication system”), it is required to avoid interference occurred between a service area of an own station and a service area of another station using the same frequency as that of the first-mentioned station.
Generally speaking, in a fixed communication, such a system capable of avoiding interference with another service area by way of antenna directivity of both a base station and a subscriber station is employed. On the other hand, in a mobile communication, there are provided a system in which when a base station is initiated, all of wireless carriers (radio carriers) allocated to the own stations are monitored to determine a use frequency, and also another system in which an occurrence of interference is sensed by measuring a bit error rate and the like of a reception signal, and then an interference packet is retransmitted and also a communication channel is changed.
However, in the system capable of realizing the two-dimensional fixed communication service by the base station installed on the ground, typically known as the P-MP fixed communication system, there is such a limitation that the occurrence of interference is avoided only by way of the antenna directivity due to complex geographical features present within the service area. Also, since the transfer band of the fixed communication system is wider than that of the mobile communication system, it is not preferable to avoid the occurrence of interference by arranging a large number of wireless carriers (like as the mobile communication) in view of effective frequency utilizations. Furthermore, considering storages of exclusively used line services, it is impossible to regularly block the base station so as to detect the occurrence of interference.
Also, as to the system for detecting the occurrence of interference by employing the bit error rate of the reception signal, since the reception side cannot recognize the presence of the channel addressed to the own station before receiving this channel in the TDM line, even if interference happens to occur, the reception side cannot detect the occurrence of this interference.
In a TDMA type wireless communication system for executing a reallocation control and a rearrangement control, reliability of a control line through which a base station communicates with a subscriber station so as to execute the above-explained controls may adversely influence performance of the entire wireless communication system. In the reallocation control, a total number of allocation time slots are changed during communication in response to a change in a traffic. In the rearrangement control, time slot arrangements are changed. As a consequence, the error correction capability of the control line is emphasized, and also the same data is transferred by using the plural frames. However, in the fixed communication system, since the interference occurs in a fixed and periodical manner, the above-explained measures cannot become effective.
Moreover, in such a communication system that the bit error rate of the reception signal is measured so as to sense the occurrence of interference, and then the interference packet is retransmitted and/or the communication channel is changed, there is a certain possibility that the occurrence of such interference is erroneously detected due to adverse influences caused by variations in space propagation losses and also spike-like noise such as shadowing and a thunderbolt. When such interference erroneous detections occur many times, there is such a problem that since the interference is unnecessarily avoided and also the data resend control is carried out, the line storage capabilities would be reduced.
The present invention has been made to solve such a problem, and therefore, has an object to provide a TDMA communication system, a base station apparatus for executing TDMA communication, and a subscriber station apparatus, capable of reallocating/rearranging communication time slots among a base station and subscriber stations, while avoiding an occurrence of interference.